Herpes simplex virus type l (HSV-l) is an increasing health problem with about 80% of the U.S. population currently infected with this virus. HSV- l infects the body surface at mucocutaneous junctions and is transported to nerve cell bodies in the innervating ganglia. It is in these cells that the transcriptional pattern of the virus is radically altered and only one locus, the Latency Associate Transcription Unit (LATU), is transcribed. At this point a "Latent" infection is established and infectious virus cannot be directly recovered from the tissue. Periodic reactivation of latency leads to recurrent disease including relatively benign "cold sores", recurrent keratitis which is responsible for over 240,000 cases of blindness in the U.S. per year, and fatal encephalitis. At the moment the molecular mechanisms which underlie this complex interaction with the host's nervous system are almost completely unknown. Two viral factors which modulate infection of neurons have recently been identified: the iCP34.5 gene which is required for replication in neurons but not other cells, and the LATU which shuts off viral gene expression in neurons, but not other cells. A murine model of herpetic disease in which all phases of infection are quantifiable will be employed in conjunction with methods to dissociate, separate and sort ganglion cells to address fundamental questions about the interaction of HSV-l with neurons. A panel of defined HSV-l genetically engineered mutant and wild type viral strains will be employed to determine: l) The cell types and numbers which support acute viral replication: 2) The fate of these cells. That is, which cells become latently infected and the number of viral genomes these cells contain. 3) The biochemical basis for the replication defective phenotype of ICP34.5 null mutants in ganglion cells in vivo, and in non permissive cells in vitro. 4) The biochemical basis for the reduced establishment phenotype of null mutants at the LATU locus. 5) The role of the LAT intron in establishment of latency, and it's interaction with ICPO. These experiments will add greatly to our knowledge of the regulation of HSV-l genes in neurons in vivo, with the goal of developing new strategies to prevent and treat herpetic disease.